System and method for vehicle power management

ABSTRACT

A power management system for a vehicle having wheels and an electric machine operable to provide torque to drive at least one of the wheels includes a first energy storage system capable of supplying power to operate the electric machine. The system also includes a second energy storage system capable of supplying power directly to at least one vehicle load at a lower voltage than the first energy storage system. A voltage conversion device is operable to reduce a voltage of the power supplied by the first energy storage system to the lower voltage to charge the second energy storage system when the vehicle is in a key-off state.

TECHNICAL FIELD

The present invention relates to a system and method for vehicle powermanagement.

BACKGROUND

Increasingly, vehicles are being powered by an electric motor, eitherexclusively, or in conjunction with another power source, such as aninternal combustion engine. In such vehicles, a high-voltage electricalpower source—e.g., a high-voltage battery—is used to power the electricmotor and other high voltage loads within the vehicle. In addition tothe high voltage battery, a hybrid electric or electric vehicle may alsohave a low-voltage battery, which may be used to power vehicle lighting,engine cooling fans, heated seats, and/or other low-voltage loads. Itmay be necessary to provide power to some of the low-voltage loads whenthe vehicle is not operating—i.e., when the vehicle is in a “key-off”state.

In order to maintain the low-voltage electrical loads during the key-offstate, it is necessary to provide a low-voltage power source having anappropriate capacity. The greater the level of the low-voltage loads,and the longer it is desired to maintain those loads, the greater therequired capacity of the low-voltage power source. For example, it maybe desirable to supply power to the key-off low-voltage loads forseveral weeks while the vehicle is idle and not being driven. In such acase, the capacity of the low-voltage power source must be relativelyhigh.

Because there is generally a relationship between the capacity of abattery and its physical dimensions, packaging a low-voltage battery ofan appropriate capacity can be problematic. In particular, theunder-hood compartments of vehicles today, which are already closelypacked, may not have the space necessary to accommodate a low-voltagebattery. Therefore, it may be necessary to choose alternative locationsfor the battery. This can undesirably increase the complexity of thewiring within the vehicle, as well as vehicle cost. Therefore, it wouldbe desirable to be able to supply power to low-voltage key-off vehicleloads for a desired amount of time, while keeping the size of thelow-voltage power source small enough to be located under the hood orwithin close proximity thereof.

SUMMARY

Embodiments of the invention include a power management system for avehicle having wheels and an electric machine operable to provide torqueto drive at least one of the wheels. The power management systemincludes a first energy storage system capable of supplying power tooperate the electric machine. A second energy storage system is capableof supplying power directly to at least one vehicle load at a lowervoltage than the first energy storage system. A voltage conversiondevice is operable to reduce a voltage of the power supplied by thefirst energy storage system to the lower voltage to charge the secondenergy storage system when the vehicle is in a key-off state.

Embodiments of the invention also include a power management system fora vehicle having wheels and an electric machine operable to providetorque to drive at least one of the wheels. The power management systemincludes a high voltage battery for supplying power to operate theelectric machine. A low-voltage battery is operable to supply power toat least one vehicle load, and a converter is operable to reduce thevoltage of power received from the high voltage battery and charge thelow-voltage battery when the vehicle is in a key-off state.

Embodiments of the invention further include a method of powermanagement for a vehicle having wheels and an electric machine operableto provide torque to drive at least one of the wheels. The vehicle alsoincludes a high voltage battery for providing power to the electricmachine, and a low-voltage battery. The method includes reducing thevoltage of the power from the high voltage battery during a vehiclekey-off state, and charging the low-voltage battery with the reducedvoltage power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic view of a vehicle including a powermanagement system in accordance with embodiments of the presentinvention; and

FIG. 2 is an electrical schematic illustrating in detail the powermanagement system shown in FIG. 1.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

FIG. 1 shows the vehicle 10 having a system 12 (see FIG. 2) inaccordance with embodiments of the present invention. The system 12includes a first energy storage system, which in the embodiment shown inFIG. 1, is a high-voltage battery 14. The HV battery 14 is operable tosupply high-voltage power to an electric machine 16. The electricmachine 16 is shown as a “motor”, but may be, for example, a combinationmotor/generator. The motor 16 receives power from the battery through ahigh current fuse box (HCFB) 18, and provides torque to drive one ormore pairs of vehicle wheels 20, 22, 24, 26. Through the HCFB 18, the HVbattery 14 also provides power to a voltage conversion device, or DC/DCconverter 28, and various other high-voltage loads through outputs 30,32, 34.

The converter 28 reduces the voltage of the high-voltage power suppliedby the HV battery 14, and provides a low-voltage output, for example, toa second energy storage system, such as a low-voltage battery 36. Asused herein, the distinction between high voltage power and low-voltagepower is the difference between voltages in the range of 50 volts orless for low-voltage power, and 100 volts or more for high voltagepower. Output from the converter 28 can be used to charge the LV battery36. As explained in more detail below, this allows the system 12 to usethe HV battery 14 to keep the LV battery 36 charged during extendedperiods of vehicle key-off. The LV battery 36 is capable of supplyingpower to at least one low-voltage vehicle load, for example, through afuse box, such as the power distribution box (PDB) 38, for distributionto one or low-voltage loads.

The system 12 also includes a control system, which is shown in theembodiment in FIG. 1 as a battery electronics control module (BECM) 40.Although it is shown as a single controller, the BECM 36 can be part ofa larger control system, connected to other controllers, for example,through a controller area network (CAN). The BECM 36 helps to controlthe converter 28, and therefore, charging of the LV battery 36 by theconverter 28.

FIG. 1 also shows a number of components and their possible locations inthe vehicle 10 if a system such as the system 12 is not used. Forexample, as explained above it is possible to charge the LV battery 36with the converter 28, which steps down the voltage of power supplied bythe HV battery 14. This means that a low-voltage battery, such as the LVbattery 36, can have a lower capacity than a battery which cannot becharged by an onboard HV battery. In order to maintain several weeks ofpower to supply key-off low-voltage loads, a low-voltage battery 42 ofhigher capacity than LV battery 36 may need to be employed. Because ithas a higher capacity, the battery 42 may be significantly larger thanthe LV battery 36. Therefore, it may not be possible to locate thebattery 42 in a desired location such as under the hood 44, but rather,it may be necessary to locate it in another available space, such astrunk space 46.

Locating a battery, such as the battery 42 in the trunk 46 presents anumber of problems, including an increase in complexity of theelectrical system and cost of the vehicle. One problem is that vehicleoperators will still require the ability to access their low-voltagebattery to use the battery to jumpstart another vehicle. When thebattery is located in the trunk 46, a positive battery post 48 must beadded near the front of the vehicle such that it is accessible under thehood 44. This requires long and expensive conducting wires 50 to be runfrom the front of the vehicle 10 to the trunk 46 to connect with thebattery 42. In addition, a body control module (BCM) 52, which acts likea “smart junction box” and may be required to connect low-voltage loadsto the battery 42. A current sensor 54 is attached to the battery 42 formonitoring the current output of the battery 42. Another downside ofhaving the low-voltage battery 42 located in the trunk 46, is that theBECM 40 (shown as BECM 40′) and the converter 28 (shown as converter28′) may also need to be located in the trunk 46 near the battery 42,further requiring additional electrical cabling 55.

FIG. 2 shows a schematic representation of the system 12. In particular,it shows the location of the converter 28 pursuant to embodiments of thepresent invention, and also the location of the converter 28′ withoutimplementation of the present invention. As shown in FIG. 2,high-voltage contactors 58, 60, 62 are disposed between the HV battery14 and the converter 28′. The contactors 58, 60, 62 are the main systemcontactors, which allow the HV battery 14 to supply power tohigh-voltage loads 64, which represent any number of high-voltagevehicle loads, including the load of the traction motor 16. Contactors58, 60 are positive contactors, and contactor 62 is a negativecontactor, and each of these contactors is open when the vehicle 10 isin the key-off state. This means that the converter 28′ does not receivea supply of high-voltage power and cannot charge the LV battery 36during a key-off state.

Conversely, with the configuration of the present invention, theconverter 28′ is moved to a position on the other side of the contactors58, 60, 62—see converter 28—toward the HV battery 14. This allows theconverter 28 to receive high-voltage power directly from the HV battery14 without the need to close the main system contactors 58, 60, 62. Asan alternative, the converter, such as the converter 28 could beconnected to a separate set of contactors, shown in phantom ascontactors 66, which would allow the converter 28 to be connected to theHV battery 14 without the need to close the main system contactors 58,60, 62.

Another advantage to moving a converter, such as the converter 28, tothe to the HV battery side of the main contactors, is that a vehicle,such as the vehicle 10, can be started in situations where it would nototherwise be possible. For example, if a low-voltage battery, such asthe LV battery 36, has a failed cell or cells, its voltage output may besubstantially below its nominal voltage rating—e.g., 12 volts. If thevoltage available from a LV battery is too low, it may not be enough tostart the vehicle. Specifically, the low voltage power supply may not beable to close the main system contactors, which is necessary tofacilitate high voltage power supply to a motor, such as the motor 16.

With embodiments of the present invention, a converter, such as theconverter 28, can operate in a key-off state as described above, and canalso operate as soon as the vehicle is in a “key-on” state—i.e., as soonas the vehicle operator has turned the ignition to “on”. In this way,high-voltage power is stepped-down by the converter and supplied to thelow-voltage loads; the main system contactors can then be closed and thevehicle started, even though the low voltage battery could not supplythe required power. The same is true even if a low voltage battery wascompletely drained and was unable to hold the charge it received fromthe high voltage battery during the key-off state.

In addition to the elements that are also shown in FIG. 1, FIG. 2 showsa service disconnect 68, which can be used to open the circuit betweenthe HV battery 14 and the converter 28 in case the converter 28 requiresservice. Such a disconnect may be particularly important when theconverter 28 is located on the battery side of the main systemcontactors 58, 60, 62. A current sensor 69 is also shown as part of thecircuit, and can be used, for example, to provide information to theBECM or battery controller 40.

In the embodiment shown in FIG. 2, the vehicle 10 is a plug-in hybridelectric vehicle (PHEV), and includes a charger 70 that is configured toreceive power from a source external to the vehicle 10 to charge the HVbattery 14. Contactors 72, 74 are configured to close upon theoccurrence of certain events, such as the connection of an externalpower source to the charger 70. The dashed line 76 shows an alternativeconfiguration for a non-plug-in hybrid vehicle, which does not includean external charger, such as the charger 70. As noted above, it isdesirable to be able to charge a low-voltage battery such as the LVbattery 36 during key-off periods so that the low-voltage battery canhave a reduced capacity and a smaller package size. This may beparticularly important in a PHEV, because use of a charger, such as thecharger 70 may actually drain a low-voltage battery, such as the LVbattery 36, at the same time it charges the HV battery 14.

In situations where charging a PHEV through an external charger, such asthe charger 70, drains a low-voltage battery, even a low-voltage batteryof relatively high-capacity, and therefore an inconveniently large size,may not be able to maintain the required key-off loads for the requiredperiod of time. Therefore, it has been common in such cases to include asmall DC/DC converter within the external charger itself This allows areduction of the voltage of some of the power provided by the externalpower source so that the low-voltage battery could be charged at thesame time the high-voltage battery was charged. Having a secondconverter, even a small one within an external charger, adds complexityand cost to the vehicle. Therefore, with embodiments of the presentinvention, a charger, such as the charger 70, can be“converterless”—i.e., it can be a relatively simple device with nointernal DC/DC converter.

In the system 12 shown in FIG. 2, the battery controller 40 isconfigured to activate the converter 28 to charge the LV battery 36during the vehicle key-off state, upon the occurrence of at least onepredetermined event. For example, the controller 40 may be configured toactivate the converter 28 to charge the LV battery 36 at somepredetermined frequency during the key-off state. Thus, the“predetermined event” for charging the LV battery 36 could be thepassing of some predetermined amount of time (the frequency interval)since the last time the LV battery 36 was charged during the samekey-off state.

Another event that could constitute a “predetermined event” and triggerthe activation of the converter 28 to charge the LV battery 36 is thestate of charge (SOC) of the LV battery 36 dropping below somepredetermined level. In at least some embodiments, the controller 40could monitor the state of charge of the LV battery 36, for example, atsome predetermined frequency during the key-off state, and then activatethe converter 28 to charge the LV battery 36 when the SOC has droppedbelow the predetermined charge level. As shown by the dashed line 78,the controller 40 is in direct communication with the converter 28, andcan provide signals to activate and control the converter 28. Inembodiments where dedicated contactors, such as the contactors 66 arerequired to be closed in order for the converter 28 to be connected tothe HV battery 14, the controller 40 can also be configured to controlthe contactors 66, either directly, or through part of a larger controlsystem.

Although the above illustrations have been described in terms of systemhardware, and a control system used to activate and control variousdevices, embodiments of the present invention include a method of powermanagement for a vehicle such as the vehicle 10. Such a method mayinclude, for example, reducing voltage of power from a high-voltagebattery during a vehicle key-off state, and charging a low-voltagebattery with the reduced voltage power. The step of charging thelow-voltage battery can be repeated at some predetermined frequencyduring the same key-off state. It may also be repeated at any time whena state of charge of the low-voltage battery drops below somepredetermined charge level. In such embodiments, the SOC of thelow-voltage battery can be periodically monitored to ensure that it doesnot drop below the predetermined charge level.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

What is claimed is:
 1. A power management system for a vehicle havingwheels and an electric machine operable to provide torque to drive atleast one of the wheels, the power management system comprising: a firstenergy storage system capable of supplying power to operate the electricmachine; a second energy storage system capable of supplying power to atleast one vehicle load at a lower voltage than the first energy storagesystem; and a voltage conversion device operable to reduce a voltage ofthe power supplied by the first energy storage system to the lowervoltage to charge the second energy storage system when the vehicle isin a key-off state.
 2. The power management system of claim 1, furthercomprising a control system including at least one controller, thecontrol system being configured to activate the voltage conversiondevice to charge the second energy storage system during the key-offstate when at least one predetermined event occurs.
 3. The powermanagement system of claim 2, wherein the at least one predeterminedevent includes the second energy storage system dropping below apredetermined state of charge.
 4. The power management system of claim2, wherein the at least one predetermined event includes the passing ofa predetermined amount of time since the last time the second energystorage system was charged by the voltage conversion device during thesame key-off state.
 5. The power management system of claim 2, furthercomprising a set of electrical contactors disposed between the firstenergy storage system and the voltage conversion device, the controlsystem being configured to close the electrical contactors uponoccurrence of the predetermined event.
 6. The power management system ofclaim 1, wherein the voltage conversion device is directly connected tothe first energy storage system.
 7. The power management system of claim1, further comprising a voltage converterless charger configured toreceive power from a source external to the vehicle and to supply powerto the first energy storage system.
 8. The power management system ofclaim 1, wherein the vehicle further includes a set of electricalcontactors disposed between the first energy storage system and theelectric machine, the voltage conversion device being further operableto close the electrical contactors to facilitate power transfer betweenthe first energy storage system and the electric machine when thevehicle is in a key-on state.
 9. A power management system for a vehiclehaving wheels and an electric machine operable to provide torque todrive at least one of the wheels, the power management systemcomprising: a high voltage battery for supplying power to operate theelectric machine; a low-voltage battery for supplying power to at leastone vehicle load; and a converter operable to reduce the voltage ofpower received from the high voltage battery and charge the low-voltagebattery when the vehicle is in a key-off state.
 10. The power managementsystem of claim 9, further comprising a controller configured toactivate the converter to charge the low-voltage battery at apredetermined frequency during the key-off state.
 11. The powermanagement system of claim 9, wherein the converter is directlyconnected to the high voltage battery.
 12. The power management systemof claim 9, further comprising a voltage converterless chargerconfigured to receive power from a source external to the vehicle and tosupply power to the high voltage battery.
 13. The power managementsystem of claim 9, further comprising a controller configured toactivate the converter to charge the low-voltage battery when thelow-voltage battery drops below a predetermined state of charge duringthe key-off state.
 14. The power management system of claim 9, furthercomprising a controller configured to activate the converter to chargethe low-voltage battery when at least one predetermined event occursduring the key-off state.
 15. The power management system of claim 9,wherein the vehicle further includes a set of electrical contactorsdisposed between the high voltage battery and the electric machine, theconvertor being further operable to close the electrical contactors tofacilitate power transfer between the high voltage battery and theelectric machine when the vehicle is in a key-on state.
 16. A method ofpower management for a vehicle having wheels and an electric machineoperable to provide torque to drive at least one of the wheels, a highvoltage battery for providing power to the electric machine, and alow-voltage battery, the method comprising: reducing voltage of thepower from the high voltage battery during a vehicle key-off state; andcharging the low-voltage battery with the reduced voltage power.
 17. Themethod of claim 16, further comprising repeating the step of chargingthe low-voltage battery with the reduced voltage power at apredetermined frequency during the same key-off state.
 18. The method ofclaim 16, wherein the step of charging the low-voltage battery with thereduced voltage power is performed when a state of charge of thelow-voltage battery is below a predetermined state of charge during thekey-off state.
 19. The method of claim 18, further comprising monitoringthe state of charge of the low-voltage battery at a predeterminedfrequency during the key-off state.
 20. The method of claim 16, whereinthe step of charging the low-voltage battery with the reduced voltagepower is performed upon the occurrence of at least one predeterminedevent.